util/find-doc-nits: do not check files in submodules in check_env_vars

util/find-doc-nits: do not check files in submodules in check_env_vars

The reports about undocumented environment variables coming from files
in submodules are superfluous;  get the list of directories
from .gitmodules and exclude them from processing.

Resolves: https://github.com/openssl/openssl/issues/28109
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28378)